It actually depends on what you're looking for. As we know, the golden has 10% more attack and the mystic one cannot have any status ailment. I'd personally go for the golden one, because it's not very often my pokémon get paralyzed or poisoned.

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"Do you know it? The legend of the Battle Pyramid? When there comes a confident trainer reaching for the golden pinnacle, there shall appear a man among men. Don't know that legend? Of course not! I just made it up! What's it supposed to mean? That, my friend, I can't say!"